Field of the Invention
The present invention relates to an optical device wafer processing method for dividing an optical device wafer into individual optical device chips along a plurality of crossing division lines, the optical device wafer being composed of a sapphire substrate and a light emitting layer formed on the front side of the sapphire substrate, the light emitting layer being partitioned by the division lines to define a plurality of separate regions where a plurality of optical devices are formed.
Description of the Related Art
In an optical device fabrication process, a light emitting layer (epitaxial layer) composed of an n-type gallium nitride semiconductor layer and a p-type gallium nitride semiconductor layer is formed on the front side of a substantially disk-shaped sapphire substrate. The light emitting layer is partitioned by a plurality of crossing division lines to define a plurality of separate regions where a plurality of optical devices such as light emitting diodes and laser diodes are formed, thus constituting an optical device wafer. The optical device wafer is cut along the division lines to thereby divide the plural separate regions where the optical devices are formed from each other, thus obtaining individual optical device chips corresponding to the optical devices.
Cutting of the optical device wafer along the division lines is usually performed by using a cutting apparatus called a dicing saw. This cutting apparatus includes a chuck table for holding a workpiece, cutting means for cutting the workpiece held on the chuck table, and feeding means for relatively moving the chuck table and the cutting means. The cutting means includes a spindle, a cutting blade mounted on the spindle, and a drive mechanism for rotationally driving the spindle. The cutting blade is composed of a circular base and an annular cutting edge mounted on one side of the circular base along the outer circumference thereof. The cutting edge is formed by performing electroforming to bond diamond abrasive grains having a grain size of approximately 3 μm, for example, to the base. The cutting edge has a thickness of approximately 20 μm.
However, since the sapphire substrate constituting the optical device wafer has high Mohs hardness, cutting of the sapphire substrate by the cutting blade is not always easy. Further, since the cutting edge of the cutting blade has a thickness of approximately 20 μm, each division line separating the devices from each other must have a width of approximately 50 μm. As a result, the ratio of the area of all the division lines to the area of the front side of the optical device wafer is increased to cause a reduction in productivity.
As a method of dividing the optical device wafer along the division lines to solve the above problem, there has been proposed a method including the steps of applying a pulsed laser beam having an absorption wavelength to the sapphire substrate along the division lines to thereby form a laser processed groove along each division line by ablation and next applying an external force to the wafer along each division line where the laser processed groove is formed as a break start point, thereby breaking the wafer along each division line (see Japanese Patent Laid-Open No. 1998-305420, for example).
However, when the laser beam is applied along each division line formed on the front side of the sapphire substrate constituting the optical device wafer to thereby form the laser processed groove, the periphery of each optical device such as a light emitting diode may be ablated to produce a fused material called debris, which adheres to the optical devices, causing a reduction in quality of each optical device.
There has been disclosed in Japanese Patent No. 3408805 a processing method for solving the above problem. This processing method includes the steps of applying a laser beam having a transmission wavelength to the sapphire substrate along the division lines from the back side of the sapphire substrate where the light emitting layer (epitaxial layer) is not formed, in the condition where the focal point of the laser beam is set inside the sapphire substrate, thereby forming a modified layer inside the sapphire substrate along each division line, and next dividing the sapphire substrate along each division line where the modified layer is formed to be reduced in strength.
However, when the modified layer is formed inside the sapphire substrate along each division line, the periphery of each optical device is surrounded by the modified layer to cause a reduction in die strength of each optical device. Furthermore, the sapphire substrate cannot be vertically divided from the back side to the front side.
There has been disclosed in Japanese Patent Laid-Open No. 2014-221483 a laser processing method for solving the above problem. This laser processing method includes the steps of setting the numerical aperture (NA) of a focusing lens for focusing a pulsed laser beam so that the value obtained by dividing the numerical aperture (NA) of the focusing lens by the refractive index (N) of a single crystal substrate falls within the range of 0.05 to 0.2, and next applying the pulsed laser beam focused by the focusing lens to the single crystal substrate in the condition where the focal point of the pulsed laser beam is set near one side of the single crystal substrate, thereby forming a shield tunnel extending (grown) between the focal point and the other side of the single crystal substrate where the pulsed laser beam has entered, wherein the shield tunnel is composed of a fine hole and an amorphous region formed around the fine hole for shielding the fine hole.